This invention relates to pressure transmitters for use in industrial process plants. In particular, this invention relates to isolators used in pressure transmitters.
Isolators use a relatively inert isolator liquid that couples pressure from a corrosive process fluid to a delicate pressure sensor. A flexible isolator diaphragm separates the corrosive process fluid from the isolator liquid, typically silicone oil. Isolator tubes or isolator passageways that are filled with the oil are used to couple pressure from the isolator diaphragm to the pressure sensor. Isolators prevent process fluids from contacting and interfering with the operation of the delicate pressure sensor.
Isolator tubes and passages are kept narrow to reduce the quantity of oil used in each isolator and thus reduce temperature errors due to oil expansion.
The narrow passageways, however, restrict the flow of oil and slow down the response of the pressure sensor. In particular, when excessive pressure (overpressure) is applied to a small isolator diaphragm at cold temperatures, the isolator diaphragm deflects and blocks (xe2x80x9cvalves offxe2x80x9d) the open end of an isolator passage directly under the isolator diaphragm. The isolator diaphragm acts much like a closed xe2x80x9cvalvexe2x80x9d over the tiny open end of the narrow passage. When the passage is blocked, the pressure sensor is slow to respond to the overpressure.
An isolator arrangement is desired that has a low isolator liquid volume but which does not unduly inhibit rapid response of the pressure sensor.
Disclosed is a pressure transmitter with an improved isolator system.
According to one aspect of the invention, a pressure transmitter has an isolator system that includes passages in a sensor tube and a fill tube that are filled with an isolation liquid. The sensor tube and fill tube have shaped ends that are arranged to connect together in a common port in a transmitter body. The arrangement provides a reduced cost connection arrangement having low isolator liquid volume without unduly inhibiting the speed of the isolator.
According to another aspect of the invention, the isolator system has an isolator diaphragm with a central diaphragm region overlying a central backing plate. The central backing plate has an annular groove that is filled with isolator liquid and that intersects with a passageway filled with isolator liquid. During overpressure condition, the annular groove allows free flow of isolator liquid under the diaphragm so that xe2x80x9cvalve offxe2x80x9d is avoided and the speed of the isolator is not inhibited.
The pressure transmitter provides a desired combination of small isolator diaphragm size, reduced temperature sensitivity and high speed in the operation of the isolator, particularly in cold temperatures during overpressure conditions.